It is well known that "radar" is used to detect the presence of various bodies, such as airplanes, ships, vehicles, and the like; this result being an "echo" technique achieved by transmitting a beam of electromagnetic energy, and then picking up its reflection from the foreign body. By noting the direction from which the energy had been reflected, and measuring the time interval between emission of the original radiation and collection of the reflected energy, it is possible to determine the direction and distance (range) of the body; and this gives rise to the acronym "RADAR" (Radio Detection And Ranging). While the term "radar" is actually a technique or an overall system, the term "radar" is widely used in a general sense to designate the emitted and/or reflected energy (as "radar pulse"), the wavelength of the electromagnetic radiation (radar frequency), etc.; and this practice will be followed.
At various times, it is advantageous to prevent bodies from being detected by radar; and to do this, the body must be configurated, treated, coated, etc. to prevent the radar-energy from being reflected; configurations and materials that accomplish this result being known as radar attenuators or radar absorbers.
Manu such radar absorbing materials are known; and various mechanical structures have been designed to permit the optimum use of these materials. One such structure--disclosed in U.S. Pat. No. 2,599,944--is known as a "Salisbury Screen"; and its operation may be understood from FIG. 1. In this illustration, assume that radar-energy approaches as indicated by arrow 20; and impinges perpendicularly upon a metallic sheet 22, which reflects the radar-energy. The impinging and reflected radar-energy coact to form a phenomenon known as a "standing wave," which is represented by sinusoidal waveform 24. Waveform 24 indicates the electrical distribution of the standing wave; and shows that it has a first maximum value at one-quarter of a wavelength (.lambda./4) from metal sheet 22; that maximum electricl values occur at odd quarter-wavelengths (.lambda./4, 3/4, etc.) from reflective plate 22; and that minimum electrical values occur at multiples of half-wavelengths (2/4, .lambda., etc.) from metal plate 22.
It is known that if a sheet having suitable electrical resistance is placed parallel to, and one-quarter to the radar wavelength from reflective plate 22,--i.e., at the point of maximum electrical value identified by reference character 26--this resistive sheet will absorb substantially all of the standing-wave electrical energy at that point; so that no portion of the standing wave or reflected radar energy will exist to the left of the resistive sheet positioned at location 26. This means that all of the reflected energy is absorbed; that none of it would reach the radar station that is searching for the radar-reflecting body; and that the body is thus hidden from the radar station. Thus, a Salisbury Screen can be used as a radar-absorbing structure that will prevent the detection of a body by radar.
As pointed out above, the resistive-sheet at location 26 must have specific characteristics; and these characteristics may be understood, in a general way, from the following discussion. As the electromagnetic radar wave is propagated through space, it "sees" a resistance having a value of 377 "ohms per square" (to be discussed later); and it propagates continuously through an environment having this resistance. Any time that the electromagnetic wave "sees" a different resistance, or "discontinuity," a disturbance is produced; this disturbance--depending upon its characteristics--causing the wave to be partially reflected, partially transmitted, and/or partially absorbed.
Referring back to FIG. 1, it will be seen that the impinging radar-energy "sees" a disturbance, namely metal sheet 22, that has a resistance of practically zero; this disturbance resulting in a reflected wave that coacts with the impinging wave to produce standing-wave 24.
As previously discussed, a resistive-sheet positioned one-quarter of a wavelength from metal sheet 22, will absorb energy from the standing wave 24, provided that the resistive-sheet has certain characteristics. One of these characteristics is that it should have a resistance of 377 "ohms per square," this term defining the electrical resistance of a square piece or unit area of material that is carrying electricity from one edge to the opposite edge. It will be realized that as the size of the square increases, its current-carrying width changes; but its current-carrying length changes in the same manner--so that its electrical resistance remains substantially constant, producing a substantially constant value of "ohms per square" that depends primarily on the characteristics of the material. The same relation holds for a square whose size is decreased; i.e., its current-carrying width and length both decrease.
Desired ohms-per-square resistance can be achieved in a number of ways. One simple way is to have a sheet of fabric impregnated with a suitable amount of resistive material, such as carbon. If the sheet has a resistance of 377 ohms per square, it is now called "space cloth."
When this resistive-sheet space-cloth is suitably positioned, the impinging radar does not experience any discontinuity, there is no disturbance, and therefore the sheet does not produce any reflection to be picked up by the radar station. The radar wave is transmitted through the resistive-sheet to the reflective metal-sheet 22, of FIG. 1, from where it is reflected to produce standing wave 24 as discussed above. Since the resistive-sheet is at a point of high electrical value of the standing-wave 24, an electrical current flow is produced in the plane of the sheet; and its electrical resistance quickly absorbs the energy. Thus a Salisbury screen as described above may be used as a radar-absorbing material that minimizes reflections, or as a radiation attenuator that protects bodies behind the screen from exposure to the incoming radiation.
It will be realized that the Salisbury screen discussed above is operative for one particular radar frequency (wavelength); since the resistive-sheet is placed a quater of a wavelength from the metallic reflector. It is known that radar stations frequently use a multiplicity of frequencies; and, to solve this problem, a Salisbury-type screen may be constructed as shown in FIG. 2.
Here it is assumed that the impinging radar-energy includes three different frequencies (three different wavelengths); and therefore three different resistive sheets are used--each positioned one quarter of the respective wavelength (at locations 26a, 26b, and 26c) from a metallic reflecting sheet 22a. Therefore, the composite multi-layer Salisbury-type screen of FIG. 2 acts to absorb the energy of the three-different-wavelength radar waves; this being known as a "broad bandwidth" absorber.
Much work has been done on Salisbury-type screens; and it has been found that it is quite difficult to make space-cloths that have a resistance of exactly 377 ohms per square; with the result that there is, in actuality, a limited reflection--and transmission--at each sheet. Moreover, the sheets of FIG. 2 must be properly spaced; and it has been found that the necessary spacing-structure also produces disturbing influences. Therefore, in actuality, the structure as shown in FIG. 2 does not work exactly as theoretically indicated. In addition, it has also been found that the sheets' operation is improved if they are partially "reactive," rather than being purely resistive, and if the various sheets have slightly different construction and resistances. Despite all of the above complexities, it has become possible to produce Salisbury-type screens of the type discussed in FIG. 2 that operate quite satisfactorily; these screens taking the form of flexible blankets, rigid structural material, etc.; their structural, electrical, and functional characteristics, etc., being discussed in U.S. Pat. No. 3,349,397 entitled "Flexible Radiation Attenuator" by J. R. Rosenthal.
Despite the various forms of Salisbury screens available, there is still a need for an inflatable radar-absorbing-material having a structure that is suitable for being inflated under desired conditions. A material of this inflatable type may, for example, be stored in a compacted deflated form until its use is desired; whereupon it can be inflated to quickly assume its design size, shape, and radar-absorbing characteristics. Another use for a device of this sort is a satellite that is launched in a compact form, and then suitably inflated to assume its desired size and shape.